i need help with a cryptography exception!

[C# Programming]

I have this code for encryption-decryption some data, when i run it, it makes the exception: "Padding is invalid and cannot be removed"
I did everything i found in google or forums but none helped me.
I appreciate any ideas! Please help me!
This is the Code:

using System;
using System.IO;
using System.Text;
using System.Security.Cryptography;


public sealed class Cryption
{
private RijndaelManaged Algorithm;
private MemoryStream memStream;
private ICryptoTransform EncryptorDecryptor;
private CryptoStream crStream;
private StreamWriter strWriter;
private StreamReader strReader;

private string m_key;
private string m_iv;

private byte[] key;
private byte[] iv;

private string pwd_str;
private byte[] pwd_byte;

public Cryption(string key_val, string iv_val)
{
key = new byte[32];
iv = new byte[32];

int i;
m_key = key_val;
m_iv = iv_val;

for(i=0;i
{
key[i] = Convert.ToByte(m_key[i]);
}
for(i=0;i
{
iv[i] = Convert.ToByte(m_iv[i]);
}

}


public string Encrypt(string s)
{
Algorithm = new RijndaelManaged();

Algorithm.BlockSize = 256;
Algorithm.KeySize = 256;
Algorithm.Padding = PaddingMode.PKCS7;

memStream = new MemoryStream();

EncryptorDecryptor = Algorithm.CreateEncryptor(key,iv);

crStream = new CryptoStream(memStream, EncryptorDecryptor, CryptoStreamMode.Write);

strWriter = new StreamWriter(crStream);

strWriter.Write(s);

strWriter.Flush();
crStream.FlushFinalBlock();

pwd_byte = new byte[memStream.Length];
memStream.Position = 0;
memStream.Read(pwd_byte,0,(int)pwd_byte.Length);

pwd_str= new UnicodeEncoding().GetString(pwd_byte);
return pwd_str;
}


public string Decrypt(string s)
{
Algorithm = new RijndaelManaged();

Algorithm.BlockSize = 256;
Algorithm.KeySize = 256;

MemoryStream memStream = new MemoryStream(new UnicodeEncoding().GetBytes(s));

ICryptoTransform EncryptorDecryptor = Algorithm.CreateDecryptor(key,iv);
memStream.Position = 0;
CryptoStream crStream = new CryptoStream(memStream, EncryptorDecryptor, CryptoStreamMode.Read);
strReader = new StreamReader(crStream);

return strReader.ReadToEnd();
}

}